Compressor capacity control



y 23, 1968 D. N. SHAW 3,385,508

COMPRESSOR CAPACITY CONTROL Filed March 25, 1964 2 Sheets-Sheet 1 34 fas \J) 82 31 9 .8 62 57 as K M1 FIG 5 1 5g 60M 45 m FIG. I

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114M442? aw ATTORNEY.

May 28, 1968 D. N. SHAW COMPRESSOR CAPACITY CONTROL 2 Sheets-Sheet 2Filed March 25, 1964 FIG. 2

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INVENTOR.

DAVID N. SHAW.

United States Patent ce 3,385,508 7 COMPRESSOR CAPACITY CONTROL David N.Shaw, Liverpool, N.Y., assignor to Carrier Corporation, Syracuse, N.Y.,a corporation of Delaware Filed Mar. 25, 1964, Ser. No. 354,714 3Claims. (Cl. 23022) This invention relates to control arrangements, andmore particularly to control arrangements for regulating the capacity offluid compressors.

In copending application, 'Ser. No. 198,854, filed on May 31, 1962, nowPatent No. 3,184,151 there is disclosed a control apparatus forregulatin the capacity of a reciprocating compressor by effectingloading and unloading of at least one cylinder of a multi-cylindercompressor. The control apparatus therein disclosed regulates theposition of a piston type valve disposed between the discharge andsuction sides of a compressor cylinder which in response to loadconditions imposed on the compressor interrupts communication betweenthe cylinder discharge and suction sides to load the cylinder inresponse to increased compressor load and which permits communicationbetween the cylinder discharge side and the suction side to unload thecylinder in response to decreased compressor load.

The control apparatus disclosed in copending application, Ser. No.198,854, is adjustable to accommodate different applications. However,the individual control requirements of the different applications withwhich that control apparatus may be used exceed the adjustmentcapabilities of a single control apparatus of that type necessitatingthat a number of control apparatuses having different ranges ofadjustment be stocked.

It is a principal object of the present invention to provide a new andmore versatile control arrangement for fluid compressors.

It is a further object of the present invention to provide an improvedcontrol arrangement for regulating operation of fluid compressors havingan extended range of adjustment.

It is an object of the present invention to provide a controlarrangement for fluid compressors having unique adjusting means wherebythe operational response of the compressor may be more closelydelimited.

It is an additional object of the present invention to provide a controlarrangement of the pressure type for regulating the capacity of fluidcompressors, incorporating a unique pressure differential adjustingmechanism. Other objects of the invention will be readily perceived fromthe ensuing description.

This invention relates to a control arrangement for a fluid compressorhaving suction and discharge sides with partitioning means therebetweenhaving an opening therethrough for communicating the compressordischarge side with the suction side, and pressure actuated piston meansfor interrupting the partition means opening, the combination of a bleedpassage serving to impose compressor discharge pressure against thepiston means to move the piston means relative to the partitioning meansopening, and means for controlling the passage of discharge gas throughthe bleed passage to control the piston means, the controlling meansincluding rst and second passages for communicating the bleed passagewith the compressor suction side, and valve means for regulating flow ofgas through the first passage in response to pressure conditions betweenthe bleed passage and the' piston means.

The attached drawings illustrate :a preferred embodiment of theinvention, in which FIGURE 1 is a cross-sectional view of a fourcylinder reciprocating type compressor embodying the control arrangementof the present invention;

3,385,508 Patented May 28, 1968 FIGURE 2 is a sectional view of aportion of a compressor embodying the control arrangement of the presentinvention, the components of the control being shown in the compressorunloaded condition;

FIGURE 3 is a sectional view of a portion of the compressor includingthe control arrangement of the present invention, the components of thecontrol being shown in the compressor loaded condition;

FIGURE 4 is an enlarged sectional view of a modified controlarrangement, the components of the control being shown in the compressorunloaded condition; and

FIGURE 5 is an enlarged sectional view of the modified controlarrangement, the components of the control being shown in the compressorloaded condition.

Referring to FIGURE 1 of the drawings, a fluid compressor 10 embodyingthe control arrangement of the present invention is shown. While thecontrol arrangement of the present invention is shown and described asapplied to a compressor of the reciprocating type, it is understood thatthe control arrangement is applicable to other types of compressors.

The compressor 10 is a four-cylinder compressor having cylinder banks 3and 4. Each of the cylinder banks 3 and 4 include a pair of cylinders 11having pistons 14 disposed therein. Pistons 14 are connected to thecrank shaft 5 by connecting rods 6.

A suction manifold 22 is provided for introducing low pressure gas intothe respective cylinders. A discharge manifold 23 is provided forreceiving the pressurized gas discharged from the respective cylinders.Discharge manifold 23 is provided with an outlet 7.

In the compressor 10, cylinder bank 3 is provided with a dischargemanifold check valve 24, more fully described hereinafter. Cylinder bank3 is also provided with control arrangement 40 for regulating thecapacity of compressor 10.

While compressor 10 is shown as a four-cylinder compressor having twobanks of paired cylinders 3 and 4, it is understood that additionalcylinder banks may be provided, some of all of which may be providedwith capacity controls.

Referring to FIGURES 2 and 3, the cylinder 11 i cludes a cylinder head12 secured to the compressor 10 by bolts 13. The piston 14 moves withinthe cylinder 11. A valve plate 15 is placed between the cylinder head 12and the cylinder 11. Cylinder head gasket 16 and valve plate gasket 17are disposed on opposite sides of valve plate 15. The cylindercommunicates with the cylinder head through the discharge port 18 andsuction port 19 in the valve plate 15.

Afiixed to the valve plate are a discharge valve 20 and a suction valve21. On the suction stroke of the piston, gases are drawn into thecylinder through the suction port 19 and chamber 32 from the suctionmanifold 22 through chamber 32. On the discharge stroke the compressedgas is forced into the discharge chamber 29 through the discharge port18. When the cylinder is in loaded condition, the cylinder dischargepressure will force open the check valve 24 to permit passage ofrefrigerant gas into the discharge manifold 23. Check valve 24 is urgedtoward closed position by spring 26 disposed between valve 24 andsupport 25.

Within the cylinder head is a partition 27 which divides the head into adischarge chamber 29 and a suction chamber 30. The chambers 29 and 30can be considered to be merely extensions of the discharge manifold andthe suction manifold, respectively. The partition 27 includes an opening33 to place the chambers 29 and 30 in communication with one another anda wall 31 defining the chamber 32. Chamber 32 is placed in communicationwith suction manifold 22 by internal passages (not shown) in thecompressor.

A piston type valve 34 serves to close the opening 33 in response to apredetermined pressure. The piston is urged by spring 35 to the openposition. Spring 35 is positioned in piston chamber 39 on spring guide36. Spring guide 36 is connected to the control apparatus body 45. Oneend of spring 35 abuts retaining ring 37 secured to piston 34 and theother end engages retaining lip 38 of the guide 36.

The control arrangement 40 includes body secured to the cylinder head 12by bolts 41. Gasket 42 is provided between the cylinder head 12 and thecontrol body 45 to prevent the leakage of the gases from the compressor.A pair of opposed chambers 46, 47 are formed in control body 45. Opening43 in control body 45 is disposed between chambers 46, 47. Guide 36 issecured to control body 45 opposite chamber 46. Control chamber 46includes valve seat 57.

Valve stem 56 having an outer dimension less than the dimension ofopening 43 is positioned in opening 43 with opposite ends extending intochambers 46, 47. Stem 56 is reduced at 44 to define a stop 66. Controlvalve 55 is slidably disposed on reduced end 44 of stem 56. Stop collar61 on reduced end 44 of valve stem 56 limits movement of control valve55 in a valve opening direction relative to stem 56. Stop 60 limitsmovement of valve 55 relative to stem 56 in a valve closing direction.Control spring 62 urges valve 55 into abutment with stop collar 61.

Retainer 63 is secured to the opposite end of valve stem 56. Spring 64,disposed between retainer 63 and control body 45, urges valve stem 56 ina valve closing direction. Spring 64 is capable of overriding valvecontrol spring 62.

A bleed passage 51 communicates control valve chamber 46 with dischargepassage 52. Passage 48 communicates control valve chamber 46 with pistonchamber 39. Cylinder head passage 53 communicates discharge passage 52with compressor discharge manifold 23. Interconnecting suction manifoldpassages 49 and in control body 45 and cylinder head 12 respectivelycommunicate bleed passage 51 with the suction manifold 30 throughcontrol valve chamber 46. Control valve regulates communication betweenbleed passage 51 and suction manifold 30.

A pressure differential control means 75 for regulating pressures incontrol valve chamber 46 is provided. Pressure differential controlmeans 75 includes passage 71 between suction passage 50 and controlvalve chamber 46, and closure means, for example, ball valve 72,

therefor. Spring 73 urges valve 72 in a closing direction.

Cooperating screw and nut type adjusting means 74 regulate tension ofspring 73.

A diaphragm assembly 65 is attached to the outer end of the control body45 by suitable means as by threaded portion 76. Diaphragm assembly 65includes diaphragm 66 adapted to abut valve stem 56. A spring 67, actingthrough diaphragm 66, urges valve stem 56 and valve 55 positionedthereon in a valve open direction, that is, cylinder unloaded position.Spring 67 is capable of overriding valve stem spring 64. Threaded cap 68is provided for the purpose of adjusting the bias of spring 67.Diaphragm chamber 69 in control body 45 communicates with suctionmanifold passage 50 by means of the clearance between valve stem 56 andthe wall of opening 43 in control body 45, chamber 47 and passage 59.

During operation of the compressor 10, gas is drawn, on the suctionstroke of the piston into the cylinder 11 from the suction manifold 22,through chamber 32, suction port 19 and suction valve 21. On thedischarge stroke of the piston the cylinder suction valve 21 is closedand the gas is forced through discharge port 18 and discharge valve 20into the discharge chamber 29. From the chamber 29, the gas flows by thecheck valve 24 into the discharge manifold 23 for discharge throughdischarge outlet 7.

Inasmuch as at least one of the other compressor cylinders is loaded atall times, a suitable pressure head is maintained in the dischargemanifold 23. When the cylinder is loaded, that is, opening 33 is closedby control piston 34, the discharge pressure of the loaded cylinderforces check valve 24 open to discharge pressurized gas into themanifold 23. At all other times, check valve 24 is closed. The purposeof check valve 24 is to prevent pressurized gas in discharge manifold 23from flowing into the suction manifold 30 while permitting thepressurized gas obtained from the cylinder during the loaded conditionthereof to discharge into the discharge manifold 23.

When control valve 55 communicates discharge bleed passage 51 withsuction passage 49 through control valve chamber 46, the reducedpressure in piston chamber 39 is insufficient to overcome spring 35.Accordingly, piston 34 is retracted. The compressor cylinder istherefore unloaded due to the communication of discharge chamber 29 withsuction chamber 30 by means of opening 33. Valve 72 of pressuredifferent control means 75, under the influence of spring 73, interruptspassage 71.

The reduced pressure in the piston chamber 39 is effected by bleedingthe gas from the compressor discharge manifold 23 into the suctionmanifold 30 through bleed passage 51, control valve chamber 46, andpassages 49, 50. The pressure of the compressor discharge gas is reducedbecause the rate of bleed through the passages 49, 50 into suctionmanifold 30 is more than the rate of supply through the bleed passage51. This reduced pressure is insufiicient to overcome the spring 35which maintains pistons 34 in retracted position. Discharge pressure inthe discharge manifold 23 (from the working cylinders) will closedischarge check valve 24 isolating the compressor discharge manifoldfrom the individual unloaded cylinder manifolds.

An increase in load on the compressor is reflected by an increase inpressure in the suction manifold 30. Diaphragm chamber 69 communicatingwith suction manifold 30 via passages 49, 50 and opening 43 in controlbody 45 reflects a similar pressure increase. The increased pressure inchamber 69 moves diaphragm 66 against spring 67 whereby valve stem 56,under the influence of spring 64, moves control valve 55 toward closedposition. It is understood that spring 64 is capable of overriding valvespring 62 holding control valve 55 against stop collar 61. The increasein suction manifold pressure required to permit diaphragm 66 to overcomespring 67 may be varied by adjustment of cap 68.

As control valve 55 moves toward valve seat 57 and closed position, therate of bleed of discharge gas from bleed passage 51 into the suctionmanifold 30 through valve chamber 46 decreases with a resultant increasein pressure in control valve chamber 46. As noted valve 72 of pressuredifferential control means is closed. Accordingly, valve 55 aloneregulates the rate at which discharge gas is bled into suction manifold30.

As control valve 55 nears closed position, the build up of pressure invalve chamber 46 overcomes the force of valve control spring 62. Valve55 is rapidly moved in a closing direction relative to valve stem 56 andsnaps shut to interrupt the bleed of discharge gas through valve chamber46 into suction manifold 30. The increased pressure in piston chamber 39moves piston 34 against the urging of spring 35 to close bypass 33thereby loading the cylinder.

At a predetermined valve chamber pressure, the force of spring 73holding'valve 72 of pressure differential control means 75 closed isovercome to uncover passage 71 and bleed gas from the control valvechamber 46 into the suction manifold 30. By this arrangement apredetermined piston closing pressure may be maintained in valve chamber46 and piston chamber 39. It is understood that the force of spring 73and thus the control pressure in valve chamber 46 may be regulated byadjustment of screw means 74.

A decrease in compressor load is evidenced by a decrease in pressure insuction manifold 30. Diaphragm chamber 69, in communication with thesuction manifold through pasages 49, 50, opening 43 in control body 45,chamber 47 and passage 59, reflects a similar decrease. The diaphragm66, in response to a decrease in suction manifold pressure, moves valvestem 56 in a valve opening direction against the force of spring 64.Control valve 55, under the influence of pressure in control valvechamber 46 remains closed. Movement of valve stem 56 brings stop 60thereof into engagement with control valve 55. Continued movement ofvalve stem 56 in a valve opening direction is opposed by pressure incontrol valve chamber 46.

With a further decrease in compressor suction pressure, diaphragm 66,acting through valve stem 56 and stop 60, opens control valve 55. Theinitial slight opening of valve 55 permits relatively high pressure gasin valve chamber 46 to seep around control valve 55 through passages 49,59 into suction manifold 30. The seepage or leakage of relatively highpressure gas around control valve 55 into suction manifold 30 tends toreduce pressures in control valve chamber 46. Valve 72 of pressuredifferential control means 75, responsive to changes in pressure incontrol valve chamber 46, moves toward closed position to maintain thepredetermined piston closing pressure in control valve chamber 46 andpiston chamber 39.

The continued movement of control valve 55 in an opening direction bydiaphragm 66 increases the rate of bleed of relatively high pressure gasin control valve chamber 46 into the suction manifold 30. As pressur incontrol valve chamber 46 falls below the predetermined pressure settingof differential control means 75, valve 72 thereof closes to interruptthe flow of gas from control valve chamber 46 through passage 71 intothe suction manifold 30.

As pressure in control valve chamber 46, urging control valve 55 in aclosed direction, decreases, movement of control valve 55 in an openingdirection accelerates. As pressure in valve chamber 46 and accordinglyin piston chamber 39 decreases with movement of control valve 55 in anopening direction, spring retracts piston 34 to open bypass 33 andunload the compressor cylinder, and control spring 62 moves controlvalve 55 against stop 61. Reduced pressure is maintained in pistonchamber 39 by the flow of discharge gas through bleed passage 51 andcontrol valve chamber 46 into suction manifold 38 in the mannerdescribed heretofore.

Valve stem 56, positioned in opening 43 in control body 45 communicatingdiaphragm chamber 69 with suction passage 49, restricts the effectivesize of opening 43. During movement of control valve in an openingdirection, the bleed of relatively high pressure gas in control valvechamber 46 around valve 55 into suction passage 49 may result in a surgeof pressure in suction passage 49. The reduced size of opening 43effectively prevents this pressure surge from affecting diaphragm 66.

In the modification illustrated in FIGURES 4 and 5 of the drawings,wherein like numerals refer to like parts, bleed passage 80 communicateswith control valve chamber 46 through passage 71 of pressuredifferential control means 75. Connecting passages 82, 83 communicatebleed passage 80 with cylinder head passage 53 and discharge manifold23. A piston type valve 85 movably disposed in passage 71 of pressuredifferential control means 75 regulates the flow of discharge gasthrough bleed passage 80 into control valve chamber 46 in a manner to bemore particularly explained hereinafter.

Piston type valve 85 has an outer dimension slightly less than thedimension of passage 71 in pressure differential control means 75. Theouter periphery of piston type valve 85 is provided with an annulargroove 88. Passage 89 in valve 85 communicates groove 88 with con trolvalve chamber 46.

With valve 72 of pressure differential control means 75 closed tointerrupt passage 71, groove 88 of piston type valve 85 is disposedopposite bleed passage 80. It is understood that pressure in controlvalve chamber 46 urges valve 85 in an upward direction as viewed inFIGURES 4 and 5 of the drawings, valve 72 of pressure differentialcontrol means 75 limiting upward movement of valve 85. Movement of valve85 in the opposite direction is limited by control valve 55.

During operation of the compressor 10, with piston 34 retracted as shownin FIGURE 2 of the drawings, the compressor cylinder is unloaded. Anincrease in pressure in suction manifold 30 indicative of increased loadon the compressor, results in the closure of control valve 55 bydiaphragm 66 in the manner described heretofore in connection with theembodiment illustrated in FIGURES 1-3 of the drawings.

Closure of control valve 55 interrupts the bleed of high pressuredischarge gas from bleed passage through passage 89 in valve 85, chamber46 and passages 49, 50 to suction manifold 30 to increase pressure inpiston chamber 39 whereby piston 34 interrupts bypass opening 33 to loadthe compressor cylinder.

Referring particularly to FIGURE 5 of the drawings, with control valve55 closed, pressure in control valve chamber 46 increases. At apredetermined pressure spring 73, biasing valve 72 of pressuredifferential control means 75 closed, is overcome. Valve 72 opens topermit relatively high pressure gas in control valve chamber 46 to bleedthrough passage 71 and passages 49, 50 into suction manifold 30. Pistontype valve maintained in abutting relation with valve 72 by pressure incontrol valve chamber 46 moves with valve 72. Movement of valve 85offsets groove 88 of valve 85 relative to bleed passage 80 to restrictcommunication of bleed passage 80 with control valve chamber 46 throughpassage 71 of pressure differential control means 75. Valve 85 and valve72 of pressure differential control means 75 cooperate to establish andmaintain predetermined piston closing pressure in valve chamber 46. Withthe establishment of an equilibrium condition, a relatively small amountof discharge gas bleeds around valve 85 and valve 72 into suctionmanifold 30.

At a predetermined decrease in compressor load, control valve 55 isopened in the manner described heretofore in connection with theembodiment illustrated in FIGURES 1-3 of the drawings to permitrelatively high pressure gas in the control valve chamber 46 to bleedaround control valve 55 and through passages 49-50 into suction manifold30. The reduction in pressure in control valve chamber 46 permits spring35 to retract piston 34 to open bypass 33 and unload the compressorcylinder.

Referring to FIGURE 4 of the drawings, movement of control valve 55 inan opening direction reduces pressure in control valve Chamber 46. Valve72 of pressure differential control means 75, responsive to changes inpressure in control valve chamber 46, moves toward closed position.Piston type valve 85 is similarly moved to bring groove 88 thereofopposite bleed passage St). The bleed of discharge gas from passage 80through passage 89 in valve 85 and control valve chamber 46 into suctionmanifold 30 reduces pressure control valve chamber 46 whereby spring 35retracts piston 34 to unload the cylinder.

While I have described a preferred embodiment of the invention, it willbe understood that the invention is not limited thereto, since it may beotherwise embodied in the scope of the following claims.

I claim:

1. In a control arrangement for a fluid compressor having suction anddischarge sides with partitioning means therebetween having an openingtherethrough for communicating the compressor discharge side with thesuction side, and pressure actuated piston means for interrupting thepartition means opening, the combination of a bleed passage serving toimpose compressor discharge pressure against said piston means to movesaid piston means relative to said partitioning means opening, and meansfor controlling the passage of discharge gas through said bleed passageto control said piston means, said controlling means including first andsecond passages for communicating said bleed passage with saidcompressor suction side, valve means for regulating flow of gas throughsaid first passage in response to pressure conditions between said bleedpassage and said piston means, and valve means for regulating flow ofgas through said first passage valve means including a first valveoperable at a predetermined pressure to bleed gas through said firstpassage into said suction side; and a second valve operable at saidpredetermined pressure to reduce communication between said bleedpassage and said piston means.

2. In a control arrangement for a fluid compressor having suction anddischarge sides with partitioning means therebetween having an openingtherethrough for communicating the compressor discharge side with thesuction side, and pressure actuated piston means for interrupting thepartition means opening, the combination of a bleed passage serving toimpose compressor discharge pressure against said piston means to movesaid piston means relative to said partitioning means opening, and meanfor controlling the passage of discharge gas through said bleed passageto control said piston means, said controlling means including first andsecond passages for communicating said bleed passage with saidcompressor suction side, valve means for regulating flow of gas throughsaid first passage in response to pressure conditions between said bleedpassage and said piston means, and valve means for regulating flow ofgas through said second passage in response to suction side conditions,said second passage means including a valve; a movable valve stem, saidvalve being disposed on said valve stem for limited movement relativethereto; bias means urging said valve in an opening direction; andactuator means for said valve stem operable in response to apredetermined suction side pressure to move said valve stem in a closingdirection to reduce the flow of gas through said second passage toincrease pressure between said bleed passage and said piston means, adetermined increase in pressure between said bleed passage and saidpiston means overcoming said valve biasing means to rapidly move saidvalve in a closing direction relative to said valve stem to interruptthe flow of gas through said second passage to move said piston means.

3. In a compressor having a cylinder including a movable piston therein,a suction manifold, a cylinder head, a discharge manifold, a partitionin the cylinder head including an opening for communicating thedischarge and suction manifolds to unload the cylinder, and movablepiston means including a pressure chamber for closing the partitionopening, the combination comprising a restricted passage communicatingsaid discharge manifold with said pressure chamber to move said pistonmeans in a first direction to load said cylinder, means for controllingpressure in said pressure chamber including a first passage for bleedingdischarge gas from said pressure chamber into said suction manifold, andfirst valve means for regulating the bleed of gas through said firstpassage in response to pressure conditions in said pressure chamber tomaintain a predetermined pressure in said pressure chamber while saidcylinder is loaded, and means for reducing said predetermined pressureto move said piston means in a second direction to unload said cylinderand render said first valve means inoperative including a second passagefor communicating said pressure chamber with said suction manifold andsecond valve means for regulating the flow of gas through said secondpassage in response to changes in suction manifold pressures, saidsecond valve means including a valve between said second passage andsaid pressure chamber, a movable operator for said valve, said valvebeing positioned on said operator for limited movement relative thereto,means biasing said valve in an opening direction, and actuator means formoving said operator in response to changes in suction manifoldpressure, so that on a predetermined change in suction manifoldpressure, said actuator means moves said operator in a valve closingdirection whereby said valve reduces flow of gas through said secondpassage to increase pressure in said pressure chamber, a predeterminedincrease in pressure in said pressure chamber overcoming said valvebiasing means to quickly move said valve relative to said operator tointerrupt communication between said second passage and said pressurechamber.

References Cited UNITED STATES PATENTS 2,522,762 9/1950 Neeson 230312,673,025 3/1954 Labus et al 23031 2,715,992 8/1955 Wilson 230-313,021,790 2/1962 Brunson 10342 3,119,550 2/1964 West et al. 230-223,184,151 5/1965 Shaw 2303l DONLEY J. STOCKING, Primary Examiner.

MARK NEWMAN, MARTIN P. SCHWADRON,

Examiners.

W. I. KRAUSS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,385,508 May 28, 1968 David N. Shaw It is certified that error appearsin the above identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 40, "of" should read or Column 6,

line :62 before "control" insert in Column 7 line 10, after "said"insert second passage in response to suction side conditions, saidSigned and sealed this 4th day of November 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

1. IN A CONTROL ARRANGEMENT FOR A FLUID COMPRESSOR HAVING SUCTION ANDDISCHARGE SIDES WITH PARTITIONING MEANS THEREBETWEEN HAVING AN OPENINGTHERETHROUGH FOR COMMUNICATING THE COMPRESSOR DISCHARGE SIDE WITH THESUCTION SIDE, AND PRESSURE ACTUATED PISTON MEANS FOR INTERRUPTING THEPARTITION MEANS OPENING, THE COMBINATION OF A BLEED PASSAGE SERVING TOIMPOSE COMPRESSOR DISCHARGE PRESSURE AGAINST SAID PISTON MEANS TO MOVESAID PISTON MEANS RELATIVE TO SAID PARTITIONING MEANS OPENING, AND MEANSFOR CONTROLLING THE PASSAGE OF DISCHARGE GAS THROUGH SAID BLEED PASSAGETO CONTROL SAID PISTON MEANS, SAID CONTROLLING MEANS INCLUDING FIRST ANDSECOND PASSAGES FOR COMMUNICATING SAID BLEED PASSAGE WITH SAIDCOMPRESSOR SUCTION SIDE, VALVE MEANS FOR REGULATING FLOW OF GAS THROUGHSAID FIRST PASSAGE IN RESPONSE TO PRESSURE CONDITIONS BETWEEN SAID BLEEDPASSAGE AND SAID PISTON MEANS, AND VALVE MEANS FOR REGULATING FLOW OFGAS THROUGH SAID FIRST PASSAGE VALVE MEANS INCLUDING A FIRST VALVEOPERABLE AT A PREDETERMINED PRESSURE TO BLEED GAS THROUGH SAID FIRST